1. Field of the Invention
The present invention relates to an image forming apparatus and, more particularly, to an image forming apparatus for forming an image by irradiating electrons emitted by an electron-emitting device on an image forming member, in which a support member (spacer) is arranged inside an envelope.
2. Description of the Related Art
Conventionally, two types of devices, namely hot and cold cathode devices, are known as electron-emitting devices. Known examples of the cold cathode devices are surface-conduction emission (SCE) type electron-emitting devices, field emission type electron-emitting devices (to be referred to as FE type electron-emitting devices hereinafter), and metal/insulator/metal type electron-emitting devices (to be referred to as MIM type electron-emitting devices hereinafter).
A known example of the surface-conduction emission type electron-emitting devices is described in, e.g., M. I. Elinson, "Radio Eng. Electron Phys., 10, 1290 (1965) and other examples will be described later.
The surface-conduction emission type electron-emitting device utilizes the phenomenon that electrons are emitted from a small-area thin film formed on a substrate by flowing a current parallel through the film surface. The surface-conduction emission type electron-emitting device includes electron-emitting devices using an Au thin film [G. Dittmer, "Thin Solid Films", 9,317 (1972)], an In.sub.2 O.sub.3 /SnO.sub.2 thin film [M. Hartwell and C. G. Fonstad, "IEEE Trans. ED Conf.", 519 (1975)], a carbon thin film [Hisashi Araki et al., "Vacuum", Vol. 26, No. 1, p. 22 (1983)], and the like, in addition to an SnO.sub.2 thin film according to Elinson mentioned above.
FIG. 17 is a plan view showing the surface-conduction emission type electron-emitting device by M. Hartwell et al. described above as a typical example of the device structures of these surface-conduction emission type electron-emitting devices. Referring to FIG. 17, numeral 3001 denotes a substrate; and 3004, a conductive thin film made of a metal oxide formed by sputtering. This conductive thin film 3004 has an H-shaped pattern, as shown in FIG. 17. An electron-emitting portion 3005 is formed by performing electrification processing (referred to as forming processing to be described later) with respect to the conductive thin film 3004. An interval L in FIG. 17 is set to 0.5 to 1 mm, and a width W is set to 0.1 mm. The electron-emitting portion 3005 is shown in FIG. 17 in a rectangular shape at almost the center of the conductive thin film 3004 for the sake of illustrative convenience. However, this does not exactly show the actual position and shape of the electron-emitting portion 3005.
In the above surface-conduction emission type electron-emitting devices by M. Hartwell et al. and the like, typically the electron-emitting portion 3005 is formed by performing electrification processing called forming processing for the conductive thin film 3004 before electron emission. That is, the forming processing is to form an electron-emitting portion by electrification. For example, a constant DC voltage or a DC voltage which increases at a very low rate of, e.g., 1 V/min is applied across the two ends of the conductive thin film 3004 to partially destroy or deform the conductive thin film 3004, thereby forming the electron-emitting portion 3005 with an electrically high resistance. Note that the destroyed or deformed part of the conductive thin film 3004 has a fissure. Upon application of an appropriate voltage to the conductive thin film 3004 after the forming processing, electrons are emitted near the fissure.
Known examples of the FE type electron-emitting devices are described in W. P. Dyke and W. W. Dolan, "Field emission", Advance in Electron Physics, 8, 89 (1956) and C. A. Spindt, "Physical properties of thin-film field emission cathodes with molybdenium cones", J. Appl. Phys., 47, 5248 (1976).
FIG. 18 is a cross-sectional view showing a typical example of the FE type device structure (device by C. A. Spindt et al. described above). Referring to FIG. 18, numeral 3010 denotes a substrate; 3011, an emitter wiring layer made of a conductive material; 3012, an emitter cone; 3013, an insulating layer; and 3014, a gate electrode. In this device, a voltage is applied between the emitter cone 3012 and the gate electrode 3014 to emit electrons from the distal end portion of the emitter cone 3012.
As another FE type device structure, there is an example in which an emitter and a gate electrode are arranged on a substrate to be almost parallel to the surface of the substrate, in addition to the multilayered structure of FIG. 18.
A known example of the MIM type electron-emitting devices is described in C. A. Mead, "Operation of Tunnel-Emission Devices", J. Appl. Phys., 32,646 (1961). FIG. 19 shows a typical example of the MIM type device structure. FIG. 19 is a cross-sectional view of the MIM type electron-emitting device. Referring to FIG. 19, numeral 3020 denotes a substrate; 3021, a lower electrode made of a metal; 3022, a thin insulating layer having a thickness of about 100 A; and 3023, an upper electrode made of a metal and having a thickness of about 80 to 300 A. In the MIM type electron-emitting device, an appropriate voltage is applied between the upper electrode 3023 and the lower electrode 3021 to emit electrons from the surface of the upper electrode 3023.
Since the above-described cold cathode devices can emit electrons at a temperature lower than that for hot cathode devices, they do not require any heater. The cold cathode device therefore has a structure simpler than that of the hot cathode device and can be micropatterned. Even if a large number of devices are arranged on a substrate at a high density, problems such as heat fusion of the substrate hardly arise. In addition, the response speed of the cold cathode device is high, while the response speed of the hot cathode device is low because it operates upon heating by a heater.
For this reason, applications of the cold cathode devices have enthusiastically been studied.
Of cold cathode devices, the above surface-conduction emission type electron-emitting devices are advantageous because they have a simple structure and can be easily manufactured. For this reason, many devices can be formed on a wide area. As disclosed in Japanese Patent Laid-Open No. 64-31332 filed by the present applicant, a method of arranging and driving a lot of devices has been studied. Regarding applications of surface-conduction emission type electron-emitting devices to, e.g., image forming apparatuses such as an image display apparatus and an image recording apparatus, electron-beam sources, and the like have been studied.
As an application to image display apparatuses, in particular, as disclosed in the U.S. Pat. No. 5,066,833 and Japanese Patent Laid-Open Nos. 2-257551 and 4-28137 filed by the present applicant, an image display apparatus using the combination of an surface-conduction emission type electron-emitting device and a fluorescent substance which emits light upon reception of an electron beam has been studied. This type of image display apparatus using the combination of the surface-conduction emission type electron-emitting device and the fluorescent substance is expected to have more excellent characteristics than other conventional image display apparatuses. For example, in comparison with recent popular liquid crystal display apparatuses, the above display apparatus is superior in that it does not require a backlight because it is of a self-emission type and that it has a wide view angle.
A method of driving a plurality of FE type electron-emitting devices arranged side by side is disclosed in, e.g., U.S. Pat. No. 4,904,895 filed by the present applicant. As a known example of an application of FE type electron-emitting devices to an image display apparatus is a flat display apparatus reported by R. Meyer et al. [R. Meyer: "Recent Development on Microtips Display at LETI", Tech. Digest of 4th Int. vacuum Microelectronics Conf., Nagahama, pp. 6-9 (1991)].
An example of an application of a larger number of MIM type electron-emitting devices arranged side by side to an image display apparatus is disclosed in Japanese Patent Laid-Open No. 3-55738 filed by the present applicant.
Of image display apparatuses using electron-emitting devices like the ones described above, a thin, flat display apparatus receives a great deal of attention as an alternative to a CRT (Cathode-Ray Tube) display apparatus because of a small space and light weight.
FIG. 20 is a perspective view of an example of a display panel for a flat image display apparatus where a portion of the panel is removed for showing the internal structure of the panel.
In FIG. 20, numeral 3115 denotes a rear plate; 3116, a side wall; and 3117, a face plate. The rear plate 3115, the side wall 3116, and the face plate 3117 form an envelope (airtight container) for maintaining the inside of the display panel vacuum.
The rear plate 3115 has a substrate 3111 fixed thereto, on which N.times.M cold cathode devices 3112 are provided (M, N=positive integer equal to "2" or greater, appropriately set in accordance with an object number of display pixels). As shown in FIG. 20, the N.times.M cold cathode devices 3112 are arranged with M row-direction wirings 3113 and N column-direction wirings 3114. The portion constituted with the substrate 3111, the cold cathode devices 3112, the row-direction wiring 3113, and the column-direction wiring 3114 will be referred to as "multi electron-beam source". At an intersection of the row-direction wiring 3113 and the column-direction wiring 3114, an insulating layer (not shown) is formed between the wirings, to maintain electrical insulation.
Further, a fluorescent film 3118 made of a fluorescent substance is formed under the face plate 3117. The fluorescent film 3118 is colored with red, green and blue, three primary color fluorescent substances (not shown). Black conductive material (not shown) is provided between the fluorescent substances constituting the fluorescent film 3118. Further, a metal back 3119 made of Al or the like is provided on the surface of the fluorescent film 3118 on the rear plate 3115 side.
In FIG. 20, symbols Dxl to Dxm, Dyl to Dyn, and Hv denote electric connection terminals for airtight structure provided for electrical connection of the display panel with an electric circuit (not shown). The terminals Dxl to Dxm are electrically connected to the row-direction wiring 3113 of the multi electron-beam source; Dyl to Dyn, to the column-direction wiring 3114; and Hv, to the metal back 3119.
The inside of the airtight container is exhausted at about 10.sup.-6 Torr. As the display area of the image display apparatus becomes larger, the image display apparatus requires a means for preventing deformation or damage of the rear plate 3115 and the faceplate 3117 caused by a difference in pressure between the inside and outside of the airtight container. If the deformation or damage is prevented by heating the rear plate 3115 and the face plate 3117, not only the weight of the image display apparatus increases, but also image distortion and parallax are caused when the user views the image from an oblique direction. To the contrary, in FIG. 20, the display panel comprises a structure support member (called a spacer or rib) 3120 made of a relatively thin glass to resist the atmospheric pressure. With this structure, the interval between the substrate 3111 on which the multi beam-electron source is formed, and the face plate 3117 on which the fluorescent film 3118 is formed is normally kept at submillimeters to several millimeters. As described above, the inside of the airtight container is maintained at high vacuum.
In the image display apparatus using the above-described display panel, when a voltage is applied to the cold cathode devices 3112 via the outer terminals Dx1 to Dxm and Dy1 to Dyn, electrons are emitted by the cold cathode devices 3112. At the same time, a high voltage of several hundreds V to several kV is applied to the metal back 3119 via the outer terminal Hv to accelerate the emitted electrons and cause them to collide with the inner surface of the face plate 3117. Consequently, the respective fluorescent substances constituting the fluorescent film 3118 are excited to emit light, thereby displaying an image.
The above-mentioned electron beam apparatus of the image forming apparatus or the like comprises an envelope for maintaining vacuum inside the apparatus, an electron source arranged inside the envelope, a target on which an electron beam emitted by the electron source is irradiated, an acceleration electrode for accelerating the electron beam toward the target, and the like. In addition to them, a support member (spacer) for supporting the envelope from its inside against the atmospheric pressure applied to the envelope is arranged inside the envelope.
The display panel of this image display apparatus suffers the following problem.
Some of electrons emitted near the spacer strike the spacer, or ions produced by the action of emitted electrons attach to the spacer. Further, some of electrons which have reached the face plate are reflected and scattered, and some of the scattered electrons strike the spacer to charge the spacer. The orbits of electrons emitted by the cold cathode devices are changed by the charge-up of the spacer, and the electrons reach positions different from proper positions on the fluorescent substances. As a result, a distorted image is displayed near the spacer.
To solve this problem, the charge-up of the spacer is eliminated (to be referred to as charge-up elimination hereinafter) by flowing a small current through the spacer. In this case, a high-resistance film is formed on the surface of an insulating spacer to flow a small current through the surface of the spacer.
As the amount of emitted electrons by cold cathode devices increases, the charge-up elimination ability becomes poorer, and the charge-up amount depends on the intensity of an electron beam. Along with this, an electron beam emitted by a device near the spacer shifts from a proper position on the target depending on the intensity (luminance) of the electron beam. For example, in displaying a moving image, the image fluctuates.